Nucleophilic Reactivity at a =CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO2 Activation

Reaction of an amido pincer complex [(CNC)*Rh(CO)] (1) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO2)Mes*Rh(CO)] (2), which is the result of a C–C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO2. The molecular structure of 2 showed a zwitterionic complex, where the CO2 moiety is covalently connected to the former =CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO2 moiety with selected primary amines RNH2 (benzylamine and ammonia), which afforded cationic complexes [(CNC)MesRh(CO)][HRNC(O)O] (R = Bz (3), H (4)). Compounds 3 and 4 are the result of a C–N coupling between the incoming amine and the CO2 fragment covalently connected to the pincer ligand in 2, a process that involves protonation of the “CH–CO2” fragment in 2 from the respective amines. Once revealed the nucleophilic character of the =CH fragment in 1, we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C6H4-CF3) yielded neutral complexes [(CNC-CH=CHR)Mes*Rh(CO)] (R = Ph (5), 2-py (6), 4-C6H4-CF3 (7)) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp2)–H bond of the deprotonated arm in 1. This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5, which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal–ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the =CH arm of the CNCMes* ligand (making a new C–C bond) followed by the action of a protic base (HN(SiMe3)2), which is able to perform a proton rearrangement that leads to the final product 5.

P incer ligands in combination with late transition metals are one of the most utilized strategies to access highly active systems both in catalysis 1 and bond activation. 2 To this date, there have been an important number of examples of catalytic processes proceeding through metal−ligand cooperation. 3 There are relatively new catalytic processes based on pyridine-based ligands, which have the potential of undergoing an aromatization−dearomatization sequence that established the foundation of modern catalysis. 4 A relevant aspect within the context of homogeneous catalysis is concerned with the stability of the auxiliary ligands under the catalytic conditions used, which may affect the outcome or even deactivate some desired catalytic transformations. 5 Specifically, in some deprotonated lutidine-based transition metal pincer complexes, the CH arm connected to the pyridinic central core is prone to undergo C−C bond-forming reactions. For instance, a PNP*-based Cu complex (PNP* = deprotonated form of PNP; see Scheme 1 for identity of its structure) becomes methylated with MeOTf; 6 PNP* Ru 7 and Mn 8 complexes undergo reversible C−C coupling with alcohols in which the aldehydes generated upon alcohol dehydrogenation are trapped as the corresponding alkoxo complexes. PNP* Re 9 and PNN* and CNC Ru 10 complexes readily activate nitriles and different carbonyl compounds 11 by reversible C−C bond formation processes. Interestingly, Milstein et al. have recently reported the Michael addition reactions of nonactivated aliphatic nitriles to α,β-unsaturated carbonyl compounds catalyzed by PNP* Mn complexes. 12 In the same line, de Vries et al. have reported a PNN* Ru complex that catalyzes the oxa Michael addition to unsaturated nitriles. 13 DFT studies strongly indicate that in these catalytic transformations, the cooperation between the metal and the corresponding dearomatized ligand leads to the generation of metalated nitrile nucleophile species, where the release of the respective organic product involves a C−C bond breaking process. Along this line, the reactivity and catalysis of transition metal, pincer-based complexes with carbon dioxide is a hot topic. 14 In this contribution, we deal with a deprotonated CNCbased pincer in combination with an "Rh(CO)" organometallic fragment featuring a "CH" arm, namely, [(CNC)*Rh-(CO)], 15 that displays an interesting nucleophilic reactivity with some substrates such as carbon dioxide and terminal alkynes, illustrating novel reactivity patterns that have been studied by a combination of spectroscopic techniques and DFT theoretical methods.

■ RESULTS AND DISCUSSION
In general, the unsaturated CHN arm in deprotonated, neutral complexes stabilized with lutidine-based pincer ligands has a nucleophilic character, while the metal may behave as an electrophilic center in possible cooperative processes that do not involve a change in the oxidation state of the metal. 16 We recently reported the synthesis of a bis(carbene) lutidine-based rhodium complex [(CNC) Mes *Rh(CO)] (1), 15 and in this contribution, we explore the reactivity of the CH arm in 1 with a variety of electrophiles, which include CO 2 and some terminal alkynes.
Exposure of a solution of 1 in DMSO-d 6 under an atmosphere of carbon dioxide for 5 min afforded new species, which was further characterized as zwitterionic complex [(CNC-CO 2 ) Mes *Rh(CO)] (2) both in solution and in the solid state (Scheme 1). Complex 2 was not soluble in most of the organic solvents tested; however, it was soluble enough in DMSO to run a complete series of nuclear magnetic resonance (NMR) experiments in solution.
The 1 H NMR image of 2 in DMSO-d 6 showed the pyridinic signals at a low field as multiplets (H p δ( 1 H) 8.01 ppm; H m δ( 1 H) 7.73 ppm), which could have been misinterpreted as the formation of a symmetric complex. However, the lack of symmetry of 2 became evident upon inspection of its 13 C{ 1 H}-APT NMR spectrum, which showed two separated doublets (centered at δ( 13 C) 182.9 and 184.0 ppm) for the two carbenic NCN atoms. We also clearly observed a CH 2 N fragment centered at δ( 1 H) 5.45 ppm as a diastereotopic pattern, 17 but interestingly we detected the presence of a singlet at δ( 1 H) 5.97 ppm, unusually at a low field, which correlated with a signal at δ( 13 C) 71.3 ppm in the 13 C{ 1 H}-APT NMR spectrum, which we identified as the fragment "CHCO 2 " (Scheme 1). The combination of different NMR techniques, such as 1 H− 1 H COSY, 13 C− 1 H HSQC, and 13 C-APT NMR spectra of 2 ( Figures S2−S4), allowed us to identify the structure of 2, which can be described as the formal addition of CO 2 to the CH arm in 1. This assertion was further confirmed by the X-ray study on a single crystal grown from a solution of complex 2 in DMSO-d 6 . The molecular structure of complex 2 is shown in Figure 1, together with the main bond distances and angles.
In compound 2, the rhodium ion exhibits a square-planar geometry. The CNC* pincer ligand is coordinated in mer fashion via pyridine and both imidazolium entities. The fourth coordination site of Rh1 is occupied by a CO ligand, which displays the shortest coordination distance (Rh1−C1, 1.809(2) Å). The CNC* pincer ligand adopts a twist conformation, with pyridine-imidazolium dihedral angles of 50.45(9)°and 53.57(7)°, whereas both imidazolium rings are twisted 67.63(9)°to each other. Thus, insertion of the COO − unit does not seem to distort the coordination environment of the chiral atom C16, as C36 exhibits similar coordination geometries. Both enantiomers of C16 (R and S) are included in the crystal packing.
There is increasing interest in utilizing CO 2 from different catalytic perspectives. 14 Besides, there are a number of examples of reactions of lutidine-based transition metal complexes with CO 2 , which happens to react in different ways depending on the pincer structure and the metal involved. For instance, some deprotonated Ru pincer systems are able to activate CO 2 by forming a C−C bond between the corresponding pincer ligand and the electrophilic carbon atom of CO 2 , with one of the O atoms of the CO 2 coordinated to Ru, 18 a reactivity comparable to the [1,3]-addition of CO 2 to dearomatized PCP Ir 19 or PNP Re 20 complexes. There are cases in which CO 2 reacts directly with the metal, for example, in an aromatized PNP-based Ir complex 21 and an acridinebased PNP Ni complex, 22 which also incorporates CO 2 by bridging Ni pincer moieties. 23 There is a recent study where CO 2 inserts into an Ni−R bond in a phenanthroline-based pincer ligand. 24 As far as we are aware, there is only one structural motif similar to the covalent connection of CO 2 in complex 2, as shown in Figure 1, which is based on the addition of CO 2 to a double deprotonated PNP-based pincer Ni complex, although the reported structure was based on NMR data in solution. 25 Given the zwitterionic nature of neutral complex [(CNC-CO 2 ) Mes *Rh(CO)] (2), we explored the reactivity of the CO 2 fragment in 2 with benzylamine (BzNH 2 ) and ammonia. NMR monitoring of the reaction of 2 with benzylamine in a 1:1 ratio in DMSO-d 6  Multinuclear NMR characterization in the solution of complexes 3 and 4 was straightforward. A simple comparison of the 1 H NMR spectra of 3 and 4 allowed us to recognize almost the same pattern of resonances for both species. In fact, the 1 H NMR spectra of 3 and 4 were almost identical and showed a high degree of symmetry; the pyridinic protons were observed as a triplet (H p ) and a doublet (H m ) in a 1:2 ratio. We also observed two well-separated broad signals around δ( 1 H) 5.7 and 5.3 ppm (which correlated with a singlet around δ( 13 C) 54 ppm in their respective 13 C{ 1 H}-APT NMR spectra), corresponding to the CH 2 N arms of the pincer ligand, which indicated that the original CH arm in 2 became protonated in both complexes 3 and 4. The symmetry of 3 and 4 was also confirmed upon inspection of their respective 13  Keeping this situation in mind, we concluded that the anions in species 3 and 4 corresponded to the products of C−N coupling of the CO 2 fragment in 2 with the incoming amine substrates, that is, carbamates HRNCO 2 − (R = Bz (3), H (4); Scheme 2). In fact, the CH 2 fragment of the HBzNCO 2 − anion in 3 was observed at δ( 1 H) 4.70 ppm as a doublet, which correlated with a signal at δ( 13 C) 44.3 ppm in the 13 C{ 1 H}-APT NMR spectrum; moreover, we detected a signal at δ( 13 C) 158.4 ppm, which corresponded to the carboxylic carbon. In the case of complex [(CNC) Mes Rh(CO)][H 2 NC(O)O] (4), we were not able to detect the carbamate anion both in its 1 H and 13 C{ 1 H}-APT NMR spectra. However, IR data from complexes 3 and 4 showed clearly the carbamoyl carboxylic fragments as strong bands at 1640 and 1635 cm −1 for 3 and 4, respectively, along with sharp peaks terminal for the Rh−CO moieties (1981 cm −1 (3); 1982 cm −1 (4)). In addition, the cationic nature of 3 and 4 was confirmed through conductivity measurements in solution (DMSO), which gave values that fit with those shown by 1:1 electrolytes (see the Experimental Section).
The outcome of the aforementioned reactions of 2 with amines contrasts with that of reactions of some carbonyl iridium complexes with primary amines; in particular, complexes [TpIr(CO)] (Tp = tris(pyrazolyl)borate) 26 or [Ir(CO)(TTP)Cl] (TTP = meso-tetra-p-tolylporphyrinato) 27 react with primary amines and afford the corresponding hydrido carbamoyl complexes. We conclude that formation of cationic species 3 and 4 corresponded to the protonation of the −CH(CO 2 )− arm in 2 from the incoming amine substrates, concomitant with the release of the corresponding carbamate anions through a C−N coupling between the amines and the CO 2 fragment in 2.
Reactivity of [(CNC) Mes *Rh(CO)] (1) with Terminal Alkynes. Next, we shifted to the study of the reactivity of the deprotonated compound [(CNC)*Rh(CO)] (1) with some unsaturated substrates, more specifically terminal alkynes. In the first place, we reacted complex 1 (formed in situ by reaction of [(CNC) Mes Rh(CO)][PF 6 ] and KN(SiMe 3 ) 2 in THF, see the Experimental Section) with phenylacetylene, leading to the formation of new species, which was further characterized as the addition product of the alkyne on 1. Surprisingly, when starting with isolated complex 1, the reaction with PhCCH did not work properly, a situation that suggested that the presence of a base (HN(SiMe 3 ) 2 ) was essential for the observed conversion, as confirmed by DFT methods (see below). In this way, in situ formed complex 1 readily added terminal arylacetylenes ArCCH at room temperature (RT) in an unusual way, affording novel complexes characterized as [(CNC-CHCHAr) Mes *Rh-(CO)] (Ar = Ph (5), 2-py (6), 4-CF 3 -C 6 H 4 (7); Scheme 3) in good yields.
As a model, the monitoring of the reaction of complex 1 (prepared in situ) with phenylacetylene in C 6 D 6 allowed the observation of the clean formation of a new compound, which was fully characterized as complex [(CNC-CH CHPh) Mes *Rh(CO)] (5; see Scheme 3). The 1 H NMR spectrum of 5 in C 6 D 6 showed a complex pattern of resonances that reflected a nonsymmetrical molecule, where the pyridinic group remained dearomatized, defined by three multiplets detected at 7.20, 6.55, and 5.62 ppm, which correlated with signals at 118.1, 129.9, and 105.6 ppm in the 13 C{ 1 H}-APT NMR spectrum, respectively. The most striking feature of the 1 H NMR spectrum of 5 was the disappearance of the original CH proton from the deprotonated arm in 1, while the other arm remained as the original methylene backbone. Judging by 1 H NMR data, it should be mentioned that there is hindered rotation around the mesityl−nitrogen bond, so that the two sides of both rings are distinct. Additionally, we observed two  to the deprotonated CH arm of the CNC* ligand in 1. This novel structural feature was confirmed by X-ray techniques; the molecular structure of 5 is shown in Figure 2 together with the main geometrical parameters.
As in precursor complex 1, the mer coordination of the CNC* pincer ligand in 5 induced a square planar geometry of the rhodium center. Overall, geometrical features concerning dearomatization of the pyridine ring and Rh−N bond length are close to those of complex 1. 15 As shown in Figure 2, the right arm of the molecule in 5 features a covalently bound trans PhCHCH− moiety. NMR structural analysis of 5 revealed that the proton attached to the C21 imidazolyl atom in Figure  2 lies right over the electron density from the CHCHPh double bond of the alkene pendant moiety. This situation explains the intense deshielding of proton H21 (δ( 1 H) 7.65 ppm) compared to those of the remaining three CH imidazolyl protons (δ( 1 H) 6.24, 6.17, and 5.89 ppm) observed in the 1 H NMR spectrum of 5.
Multinuclear NMR data for complexes 6 and 7 follow the same pattern as those observed for 5, which indicate that all of them are isostructural. These new complexes 5−7 can be described as the products from the formal insertion of the corresponding alkyne molecule into the C(sp 2 )−H bond of the deprotonated arm in 1. Such transformations involve C−C bond formation and C−H bond activation processes. In an attempt to get insight into the possible mechanism of formation of complex 5, we carried out in situ deuterium labeling studies with monodeuterated phenylacetylene (PhC CD) in combination with complex 1. The 2 H NMR spectrum ( Figures S29 and S30) of the reaction mixture of 1 and PhC CD in a 1:1.5 molar ratio showed after 30 min a broad resonance at δ 7.82 ppm in toluene, which corresponds to the proton attached to the C33 carbon atom in Figure 2. In principle, this observation indicates a formal insertion of the alkyne into the C(sp 2 )−H bond in deprotonated complex 1; however, upon 3 h of standing, deuterium was observed to be distributed also at the CH 2 N fragment of the molecule at C13, probably due to an H/D exchange with free PhCCD.
The results reported here contrast with those shown by related rhodium/pincer systems. For instance, a lutidine-based PNP highly unsaturated rhodium complex activates terminal acetylenes by forming the corresponding vinylidene derivatives, 28 whereas a lutidine-based PNP iridium complex rather activates terminal alkynes through oxidative addition of the C(sp)−H bond to iridium, in a process that involves metal− ligand cooperation. 29 DFT Study on the Reactivity of [(CNC) Mes *Rh(CO)] (1) with Terminal Alkynes. In order to shed light into the obtained results, a DFT study on the reactivity of 1 with phenylacetylene has been carried out. The full ligand CNC Mes * has been considered in the calculations (R = mesityl). Based on preliminary studies, we propose four possible pathways involving respective C−H and CC triple bond activations of phenylacetylene mediated by complex 1 (Scheme 4).
The results of alkyne activation through paths (a) and (b) are shown in    Figure S30. The activation energy for this step is 16.5 kcal mol −1 and therefore is the lowest energetic barrier of the four proposed pathways studied (pathways a−d). This unexpected mechanism can be rationalized by inspection of the Fukui function for the removal of an electron from complex 1 shown in Figure  5, which reveals the sites on a molecule that are susceptible for electrophilic attack. 30 The blue spot at the deprotonated CH arm indicates that electron density is susceptible for an electrophilic attack at the carbon atom. The intermediate formed, d-B, is exergonic (7.1 kcal mol −1 ), and it can be stabilized by the protic base present in the reaction media (HN(SiMe 3 ) 2 ). In this situation, the approximation and further proton transfer from an HN(SiMe 3 ) 2 molecule takes place via d-TSBC BH , presenting an energetic barrier of 10.6 kcal mol −1 and leading to d-C BH . Finally, the base can abstract the proton through d-TSCD BH leading to product D BH , which can conjugate the double bonds over an extended π system, and it is strongly exergonic (−24.4 kcal mol −1 ).   In this contribution, we have shown the reactivity of a carbonyl rhodium(I) complex stabilized with a deprotonated CNC Mes * pincer ligand. The presence of an "CH" arm in the complex induces a high nucleophilic character, so reactivity with electrophiles was expected. In fact, the carbonyl complex reacted with CO 2 (g) to yield a new zwitterionic complex, which displays a CO 2 moiety covalently linked to the pincer skeleton in the former "CH" arm. This CO 2 zwitterionic adduct readily reacted with nucleophiles such primary amines (benzylamine and ammonia), which gave the protonated, cationic Rh carbonyl compound, where the corresponding anions were the corresponding carbamates [HNRCO 2 ] − (R = Bz, H), which are the products of the C−N coupling between the CO 2 fragment and the respective incoming amine. We further explored the reactivity of the deprotonated carbonyl Rh(I) complex with selected terminal alkynes ArCCH (Ar = Ph, 2-py, 4-CF 3 C 6 H 4 ), which cleanly afforded new complexes, which showed the incoming alkyne covalently connected to the CNC ligand. Both deuterium-labeling experiments and DFT studies show these compounds to be the formal products of the insertion of the incoming alkyne into the C−H bond of the arm. In particular, DFT studies showed an MLC mechanism, where the CH arm undergoes a nucleophilic attack on the alkyne affording a carbanion intermediate. The presence of a protic base is necessary as it acts as proton shuttle to achieve the formation of the final product. It should be stressed that the nucleophilicity of the CH arm of the ligand encompasses the change of the character of the N−Rh bond (amido/amino), setting in this way the basis for an MLC mechanism in which the metal does not change its oxidation state during the whole transformation.
Synthesis. All experiments were carried out under an atmosphere of argon using Schlenk and dry box techniques. Solvents were distilled immediately prior to use from the appropriate drying agents or obtained from a solvent purification system (Innovative Technologies). Oxygen-free solvents were employed throughout. CD 2 Cl 2 and (CD 3 ) 2 CO were dried using activated molecular sieves, while C 6 D 6 and toluene-d 8 were dried over a solid Na/K amalgam. Carbon monoxide was purchased from Air Liquid. Compounds [(CNC) Mes Rh(CO)][PF 6 ] and 1 were prepared according to published procedures. 15 Deuterated phenylacetylene (PhCCD) was prepared by following a recent synthetic procedure. 31 In Situ Formation of [(CNC-CO 2 ) Mes *Rh(CO)] (2).
In an oxygen-free J Young pressure NMR tube (Wilmad) were suspended crystals of 1 (25 mg, 0.04 mmol) in DMSO-d 6 (0.4 mL). The resulting red mixture was pressurized with carbon dioxide (5 bar), affording a yellow solution, which was immediately subjected to a full NMR characterization, allowing observation of the complete and clean formation of complex 2. 1 H NMR (300 MHz, DMSO-d 6  An oxygen-free J Young pressure NMR tube (Wilmad) was charged with complex 2 (25 mg, 0.04 mmol), dissolved in DMSO-d 6 (0.4 mL), and pressurized with CO 2 (5 bar). The resulting solution was frozen in liquid nitrogen for a few seconds, and then, the argon atmosphere was removed and replaced with gaseous ammonia (2  13  To a yellow solution of complex [(CNC) Mes Rh(CO)][PF 6 ] (0.10 g, 0.13 mmol) in toluene (5 mL), solid KN(SiMe 3 ) 2 (27 mg, 0.13 mmol) was added, and the resulting deep red suspension was stirred for 1 h. The mixture was filtered via a cannula, and then, pure phenylacetylene was added via syringe (13 μL, 0.12 mmol), affording a dark brown solution, which was stirred for 3 h. The volume was concentrated to ca. 2 mL by vacuum, and the addition of hexanes led to the precipitation of a dark brown solid, which was subsequently washed with hexanes and then dried under vacuum. Yield: 60 mg (71%   Crystal Structure Determination of Complexes 2 and 5. Xray diffraction data were collected with an APEX DUO Bruker (compounds 2 and 5) diffractometer, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Single crystals were mounted on a fiber and coated with a protecting perfluoropolyther oil. Data were collected at 100(2) K using ω-scans with a narrow oscillation frame strategy (Δω = 0.3°). Diffracted intensities were integrated and corrected of absorption effects by using a multiscan method using SAINT 32 and SADABS 33 programs integrated in the APEX2 package. Structures were solved by direct methods with SHELXS 34 and refined by full-matrix least squares on F 2 and SHELXL 35 programs and the WINGx package. 36 Structural Data for 2. CCDC 2142916 (2) and 1898012 (5) contain the supplementary crystallographic data for this paper. These data may be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.
Computational Details. All DFT theoretical calculations were carried out using the Gaussian program package. 37 The B3LYP-D3 method 38 in combination to the def2-SVP basis set 39 together with the corresponding core potential for Rh has been employed for geometry optimizations and vibrational frequencies. Energies were further refined by single point calculations using the M06L(SMD)/ def2-TZVP level of theory 40,41 including solvent corrections for toluene. The "ultrafine" grid was employed in all calculations. Relative energies are Gibbs free energies referred to a 1 M standard state using the approximation of Goddard et al. 42 at 25°C. The nature of the stationary points was confirmed by analytical frequency analysis, and transition states were characterized by a single imaginary frequency corresponding to the expected motion of the atoms.